Exosomes are secreted in the extracellular microenvironment by most cell types. Exosomes are enriched in proteins, lipids, mRNA, and microRNA molecules, and can act on local and distant recipient cells to mediate intercellular communication. Exosomes are implicated in the progression of a variety of diseases. In the context of infection, they have been shown to activate or inhibit the immune system, depending on the pathogen. As such, exosomes are promising for vaccination and diagnostic purposes because they are produced naturally, can be detected in and purified from bodily fluids, and their contents are disease specific and functionally active in recipient cell types. Despite their increasingly recognized involvement in mediating cell-cell communication and regulation of infection, existing techniques limit our ability to study the physiological interactions and the subsequent effects of exosomes both in vitro and in vivo. We propose to integrate the strengths of microscale technologies to develop a unique physiologically relevant lab-on-a-chip platform to enable real-time monitoring of the exosome activity during Yersinia pestis (Yp) infection. Yp is a highly infectious plague pathogen for which an FDA- approved vaccine or highly effective therapy is not available. While certain aspects of Yp virulence have been well characterized, making it an ideal model to study Gram-negative infections, the role of exosomes in infection progression of Yp remains to be elucidated. Consequently, Yp is a suitable candidate for our proposed work and the knowledge gained will provide an important foundation for studying the role of exosomes during other Gram-negative infections. Our results have demonstrated that treatment of monocytes with exosomes released by Yp-infected cells (EXi) enables them to clear out the Yp bacteria at a significantly higher rate. We have also shown that EXi induce changes in cell cycle and differentiation state of bystander nave (uninfected) monocytic cells, and also induce them to secrete specific pro-inflammatory cytokines. Based on these observations, we hypothesize that under physiological conditions EXi mediate immune responses in recipient cells, including cytokine release and immune cell migration (such as neutrophils) to the site of infection. As current knowledge about exosomes is based on in vitro assays with limited physiological relevance or hard to monitor in vivo studies, the proposed microfluidic approach will allow us to visualize and monitor exosomal transfer and also the EXi effects on recipient monocytes, including regulation of their cell cycle dynamics, and their stimulation to release pro-inflammatory cytokines that can in turn induce neutrophil migration towards the infection site.